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But he claims the amount, 25 milligrams, was small and should not have been fatal.

To secure a conviction for involuntary manslaughter prosecutors will have to prove that Dr Murray acted recklessly and created a high risk of death.

He will also be asked why he didn't tell paramedics or doctors at the UCLA hospital, where Jackson was rushed after collapsing, that he had administered propofol.

Leading US attorney Roy Black said the intensity of the two month police investigation made it inevitable that Dr Murray would be charged. He said: "I have no doubt he's going to be indicted. Just the amount of work tells us that."

The homicide ruling by the coroner's office was based on forensic tests that found propofol combined with at least two other sedatives, lorazepam and midazolam, to kill Jackson.

Lee Cantrell, director of the San Diego division of the California Poison Control System, said 25 milligrams of propofol was "not a whopping amount" but in a cocktail of other sedatives may have "pushed him over the edge".

He said: "This is horrible polypharmacy. No one will treat an insomniac like this."

Dr Murray has said he had been trying to wean the singer off propofol. He had been treating Jackson for insomnia for six weeks with 50 milligrams of the drug every night through an intravenous drip.

He had lowered the propofol dose to 25 milligrams and added lorazepam and midazolam two days prior to Jackson's death. The combination succeeded in helping the pop star sleep.

Dr Murray said he cut off the propofol completely the next day and Jackson fell asleep with just the two sedatives. But on June 25 he gave in to the singer's repeated demands for propofol.